This invention relates to a method and apparatus for preventing overheating or core meltdown in a nuclear reactor.
A nuclear reactor based on the fission of heavy atomic nuclei must be kept within a relatively slowly controllable thermal equilibrium even in a compact impulse operation. At the transition from a noncritical (nonexothermal) state into a critical state the overheating of the reactor is usually prevented by a controlled removal of the heat by exchangers which drive heat engines or by the use of nutron absorbing rods.
Absorbing rods are usually made of boron or boron containing compounds, hermetically sealed in containers made of high temperature resistant materials and welded by electron beams. Since the injection of the rods into the reactor occurs relatively slowly, powered by remotely controlled motors, the slowly varying conditions of equilibrium may be maintained as long as equilibrium exists. The velocities of the motion of the rods inward and outward are well within a wide range of an accepted safety scale allowing for mechanical interruptions or friction of the rods, such that a largest acceptable accident (GAU) is kept very small. Difficulties, however, can appear, especially in those cases when, as in Chernobyl in 1986, safety circuits fail or are eliminated. Overheating may then result because the absorbing rods are not driven into the core fast enough. Another example is the well known Three Mile Island accident which could have been prevented if faster absorbing rod insertion could have been provided.
Because the insertion of absorbing rods is relatively slow, other systems employing gases or fluids for flooding the reactor core with neutron absorbing materials have been proposed. One example is shown in Overhoff et al., U.S. Pat. No. 4,279,697 in which a controller controls valves for flooding the reactor core with aqueous gadolinium acetate which is housed in a supply vessel separate from the core. The problem with the Overhoff system is that it is partially dependent upon valves which must be opened in order to flood the reactor core. A passive link to the reactor core is provided which includes a fusible element located in a tube entering the reactor core, however, the fusible element is located so far from the core that the critical temperature in the core will be exceeded before the fusible element melts. A second problem is, that although the gadolinium acetate solution can be pumped into the core faster than boron rods can be inserted, the speed of an aqueous solution may still be too slow to prevent overheating.
A second approach is proposed in patents to Zinn, U.S. Pat. No. 2,919,236 and in Huston et al., U.S. Pat. No. 2,987,455. In the Zinn device a pipe is located within the reactor core that includes two compartments. The compartments are separated by a meltable fuse so that He.sub.3 gas stored in an upper compartment outside the core area can flow into the lower compartment, which is inside the core, absorbing neutrons and slowing the reaction. The problem with the Zinn device is that the meltable fuse is located too far from the core to react fast enough to a sudden increase in core temperature and melt effectively before the critical temperature is exceeded.
This problem is dealt with somewhat in Huston et al. which includes a number of pellets which include a dual chamber design. One chamber includes He.sub.3 gas under pressure and the other chamber is to allow for the expansion of the He.sub.3 gas into a larger volume when the fuse melts. This increases the neutron absorbing volume of the gas when the critical temperature is reached. This type of safety device, if used, is for use with a reactor that uses similarly sized fuel slugs such as a Hanford-type reactor. It is not suitable for use in a reactor which uses elongate fuel rods. Furthermore, the slugs may fail, subjecting the core to cool down at inappropriate times, or require core shutdown so that the slugs may be replaced periodically.